This invention relates to a high resolution optical system used for inspecting and observing fine pattern defects, foreign matter, etc, which appear, for example, in manufacturing processes of semiconductor devices and flat panel displays. The invention also relates to a defect inspecting apparatus that uses such a high resolution optical system.
A conventional technique, which has provided a method and an apparatus for photographing the structures of fine lines in width using an optical microscope, is disclosed in Japanese Patent Laid-Open No. 7-128595. This technique is characterized by the use of light which is linearly polarized by a polarizer positioned at about 45xc2x0 to the linear dimension of a sample. The optical delay axis of a xc2xc wavelength plate placed between the polarizer and the sample is angled optimally (25xc2x0 typically) to the main linear shape of the sample. This xc2xc wavelength plate converts the linearly polarized light to elliptically polarized light, which is then applied to the sample. This elliptically polarized light, when reflected from the sample, has a phase difference. The reflected light passes through the xc2xc wavelength plate again, then passes through a polarizer provided in a detecting light path. The light passing through the polarizer forms an image of the sample on a photoelectric conversion element. In such conventional apparatus, therefore, the phase difference caused by the sample is estimated beforehand, enabling the light set as elliptic polarized light to be converted to circularly polarized light after reflection from the sample.
In the method and apparatus for imaging structures of fine line width using an optical microscope as described, a polarizer is disposed in a lighting light path, and a linearly polarized light is passed through the polarizer. Then this linearly polarized light is converted to an elliptic polarized light through the xc2xc wavelength plate before it is applied to the sample. In such an optical system, therefore, both 0-order diffracted light reflected from the sample and higher-order diffracted light become circularly polarized light, and the ratio between the amplitudes of the 0-order diffracted light effective for forming optical images and higher-order diffracted light is the same as that of the random polarized light (the amplitude of the 0-order diffracted light is larger than that of the higher-order diffracted light). Consequently, the 0-order diffracted light and the higher-order diffracted light interfere with each other, thereby degrading the resolution of the optical image of the sample, particularly because the amplitude of the higher-order diffracted light is small, and this causes the low frequency component to be increased.
If an image sensor is used for detecting images, the light intensity is adjusted so as to prevent saturation. For example, because a cyclical fine pattern increases the diffraction angle, the contrast between patterns is small, making detected images dark. If a pattern image is detected and this detected image is processed for defect inspection, the small difference in contrast between patterns, and the dark image causes lower detection sensitivity.
The present invention provides an optical system that can control the polarization of both 0-order diffracted light used to form optical images and higher-order diffracted light, thereby detecting object patterns for inspection at a high resolution, enabling detection of finer defects.
To achieve this, the invention provides a method for observing a sample with patterns formed thereon. The method includes applying a polarized light to the sample through an objective lens, detecting the polarized light applied to and reflected from the surface of the sample through the objective lens, thereby calculating a deviation of the polarized light from the focal point on the surface of the sample in the axial direction thereof, then adjusting the height of the sample to the objective lens according to the calculated deviation from the focal point, and detecting the polarized light reflected from the surface of the height-adjusted sample through the objective lens, as well as a phase difference plate and an analyzer.
The present invention also provides another method for observing a sample with patterns formed thereon using an optical system. The method includes the steps of applying a polarized light to the sample from the optical system set on a first polarizing condition, detecting the light applied to and reflected from the surface of the sample through a phase difference plate and an analyzer to thereby obtain a first image, displaying the first image on a monitor screen, setting the optical system on a second polarizing condition according to the displayed first image, applying the polarized light to the sample while the optical system is set on the second polarizing condition, and detecting the light applied to and reflected from the surface of the sample through the phase difference plate and the analyzer, thereby obtaining the second image.
The present invention also provides an apparatus for observing a sample with patterns formed thereon. The apparatus comprises a light source for applying a polarized light to the sample through an objective lens, a focal point detector for detecting the light applied to and reflected from the surface of the sample through the objective lens, and calculating a deviation of the polarized light from the focal point on the surface of the sample in the axial direction thereof, a height adjustment device for adjusting the height of the sample to the objective lens according to the deviation from the focal point, calculated by the focal point detecting means, and a polarized light detector for detecting the light reflected from the surface of the sample through the objective lens, as well as a phase difference plate and an analyzer when the polarized light is applied from the lighting means to the sample whose height is adjusted by the height adjusting means.
In another embodiment, the present invention provides a method for inspecting defects of a sample with patterns formed thereon. The method includes the steps of applying a polarized light to the sample through an objective lens, detecting the polarized light applied to and reflected from the surface of the sample through the objective lens, as well as a phase difference plate and an analyzer, thereby obtaining an image of the surface of the sample, then comparing the obtained image with a corresponding image stored beforehand, thereby detecting defects of the sample.
In yet another embodiment, the present invention also provides a method for inspecting defects of a sample with patterns formed thereon which method includes the steps of applying polarized light to the sample from an optical system set on a first polarizing condition, detecting the light applied from the optical system and reflected from the surface of the sample through a phase difference plate and an analyzer to thereby obtain a first image, displaying the first image on a monitor screen, setting the optical system on a second polarizing condition according to the first image displayed on the monitor screen, applying polarized light to the sample while the optical system is set on the second polarizing condition, detecting the light applied to and reflected from the surface of the sample through the phase difference plate and the analyzer to thereby obtain a second image, then comparing the second image with a corresponding image stored earlier so as to detect defects of the sample.
Further, the present invention also provides apparatus for inspecting defects of a sample with patterns formed thereon, the apparatus comprising a light for applying a polarized light to the sample through an objective lens, a polarized light image detector for detecting the light reflected from the surface of the sample through the objective lens, as well as a phase difference plate and an analyzer when the polarized light is applied to the sample to thereby obtain an image of the sample, and a defect detector for comparing the image obtained by the polarized light image detector with a corresponding image stored beforehand so as to detect defects of the sample.
Further, the present invention also provides a high resolution optical system comprising an optical system for applying a polarized light to a sample, an optical part for polarization for passing a higher-order diffracted light, which is polarized and rotated by the sample, more efficiently than the 0-order diffracted light, and an optical system for detection, used for forming an image of the sample on a photoelectric conversion element with the light passing or reflected from the optical part for polarization. (For example, the high resolution optical system applies a polarized light to the sample with oscillations orthogonal to a line pattern of the sample and the optical system is provided with a polarizer disposed so as to assume the vibrating direction of 45xc2x0 to the pattern, as a transmission axis.)
This polarized light can originate from a light passing or reflected from a polarized light beam splitter. If such a light is used, a xc2xd wavelength plate and a xc2xc wavelength plate are disposed between the polarized light beam splitter and the sample, so that the xc2xd wavelength plate is rotated according to the orientation of the pattern, etc., thereby rotating the polarizing direction, and the xc2xc wavelength plate is rotated, thereby adjusting the ellipticity of polarization.
Further, the present invention allows the combination of the 0-order diffracted light used to form optical images and the direction of the higher-order diffracted light to be varied in many ways. The combination is important for making the high resolution optical system practical. The high resolution optical system is provided with functions for collecting images detected respectively by changing the polarization status and carrying out a preliminary defect inspection so as to select a polarizing condition for improving the defect inspect sensitivity. The optimal value for the polarizing condition can thus be found correctly and quickly.